Recently, as Japanese Patent Application Laid-Open No. 2002-076356 also discloses, developments of thin-film transistors (TFTs) each using a polycrystalline thin film of a transparent conductive oxide including ZnO as a main component for a channel layer have been actively performed.
Since the above-mentioned thin film can be formed as a film at a low temperature and is transparent to visible light, and it is made possible to form a flexible transparent TFT on a substrates such as a plastic plate or a film.
Furthermore, Japanese Patent Application Laid-Open No. 2003-086808 discloses that an insulating layer is formed into two-layer structure in a thin-film transistor which has a transparent semiconductor using ZnO or the like. Here, it is said that it is possible to aim to enhance crystallinity in a semiconductor layer and to reduce a defective level of an interface between a semiconductor and an insulating film by using an oxide (e.g., SiO2) on a semiconductor interface side, and using highly insulating SiNX or the like. on a gate electrode side.
In addition, a technique of using for a channel layer of a TFT a transparent amorphous oxide semiconductor film (a-IGZO) which is made of indium, gallium, zinc, and oxygen is disclosed in Nature, 488, 432, (2004). Further, it is shown that it is possible to form a flexible and transparent TFT, which shows good field effect mobility of 6 to 9 cm2V−1s−1, on a substrate such as a polyethylene terephthalate (PET) film in room temperature.
Furthermore, in Table 2 of Page 74 of Nikkei Micro Device, February 2006, there is description of using SiON for an insulating layer of a thin-film transistor which uses a-IGZO for a channel layer of a TFT.
Since oxygen defects can be generated easily and many carrier electrons are generated in a conductive transparent oxide containing ZnO as a main component, it is difficult to make electric conductivity small.
On the other hand, generally, amorphous silicon nitride (SiNx) formed by a PFCVD method is used as a gate insulating film of an amorphous silicon TFT. However, when SiNx is used as a gate insulating film in a TFT in which a conductive transparent oxide containing ZnO as a main component is used as an active layer, a large current flows between a source terminal and a drain terminal also at the time of not applying a gate voltage. In consequence, it becomes difficult to enlarge an on/off ratio of a transistor. As its main cause, as described also in Japanese Patent Application Laid-Open No. 2002-076356, it is cited that ZnO crystallinity near the interface decreases because oxygen is taken from an oxide semiconductor in an interface between a conductive transparent oxide channel layer and the gate insulating film.
In addition, as matters other than the crystallinity decrease, when using for a channel layer of a TFT the transparent amorphous oxide semiconductor film (a-IGZO) which is made of indium, gallium, zinc, and oxygen as disclosed in Nature, 488, 432, (2004), the following problems exist. That is, there is a case of generating the increase of the electric conductivity which is considered to be based on generation of defects in the interface between the conductive transparent oxide channel layer and gate insulating film. Also in this case, even at the time of not applying the gate voltage, a large current flows between the source terminal and drain terminal, and hence, it is difficult to achieve a normally-off operation of a TFT. Furthermore, it is not necessarily easy to enlarge the on/off ratio of a transistor.
Moreover, when an insulating layer has a two-layer structure of using an oxide (e.g., SiO2) on a semiconductor interface side and highly insulating SiNx or the like on a gate electrode side as disclosed in Japanese Patent Application Laid-Open No. 2002-076356, there were the following problems. That is, there were problems that hysteresis of TFT characteristics was expanded and reproducibility of TFT characteristics was poor since an interface was constructed in the insulating layer to generate defects. In addition, when a TFT was formed on a film at a low temperature and a bending test was performed, degradation of the TFT characteristics was found which was considered to be caused by interface defects generated in the insulating layer having the two-layer structure.
Furthermore, when insulating films with a high dielectric constant, which are made of oxides such as Al2O3, Y2O3, and HfO2, are formed at a low temperature of 300° C. or less, or room temperature, these insulating films with a high dielectric constant become polycrystalline substances. Then, it is usually difficult to make crystal orientation and polycrystal grain size uniform, and to improve stability and reliability. In addition, since a portion where grain size changes exists in a thickness direction at an early stage its polycrystalline structure is easy to become ununiform. Since an insulating film surface had surface morphology according to the grain size, there was a problem that an interface between a channel layer and a gate insulating film, or an interface between the gate insulating film and a gate electrode metal did not become flat.
Then, an object of the present invention is to provide a thin-film transistor having a good interface between an oxide semiconductor and an insulating layer.